has_stopped_jobs() naively checks task_is_stopped(group_leader). This
was always wrong even without ptrace, group_leader can be dead. And
given that ptrace can change the state to TRACED this is wrong even
in the single-threaded case.

Change the code to check SIGNAL_STOP_STOPPED and simplify the code,
retval + break/continue doesn't make this trivial code more readable.

We could probably add the usual "|| signal->group_stop_count" check
but I don't think this makes sense, the task can start the group-stop
right after the check anyway.
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Tejun Heo <tj@kernel.org>